Power amplifiers (PAs) are commonly used in wireless devices to amplify a signal for transmission. PAs are typically formed of multiple stages, and often include a final gain stage, commonly referred to as an output gain stage, to amplify the signal to a desired power level for its output to a load such as an antenna that radiates the amplified signal.
Wireless communication systems dictate the amount of power to be output. Different requirements may exist in different communication protocols. Many communication systems have various requirements for a handset to achieve with relation to power, efficiency, and linearity over varying signal levels. For example, a variety of communication systems, including enhanced data rates for GSM evolution (EDGE), long term evolution (LTE/4G), WiFi in accordance with an IEEE 802.11 standard, worldwide interoperability for microwave access (WiMax), code division multiple access (CDMA), and wideband-code division multiple access (W-CDMA) all have varying requirements. For example, a W-CDMA band 1 (B1) PA should be able to output approximately 30 dBm of power at an efficiency better than 40% and at a linearity of better than −40 dBc of adjacent channel leakage ratio (ACLR).
Many PAs can consume significant power in their operation. Accordingly, some PAs provide for operation at multiple supply voltage levels, where certain parts of the PA operate using a first supply voltage and other parts operate using a second, lower supply voltage. However, for certain designs of an output gain stage, it becomes difficult or impossible to operate at a low voltage that may be desired by a particular mobile platform vendor such as a cell phone vendor.